The present invention relates to improvements in storage facilities for highly chilled liquified gases. More particularly, the present invention relates to improvements in offshore terminal and submarine storage facilities for liquified energy gases, including liquified natural gas (LNG).
It has been long known to liquify gases, including natural gas, by chilling to reduce volume and thereby facilitate transportation and storage. A significant drawback stemming from the liquification and concomitant concentration of high energy gases is the vastly increased threat to safety and potential for devastation.
A liquid natural gas disaster occurred in the Cleveland, Ohio vicinity in 1944 in which hundreds of people were killed and injured. This disaster effectively terminated the use of liquified natural gas in the United States for the next twenty years.
One the other hand, liquid petroleum gases (LPG) such as propane and butane, have been in widespread uses in both rural and industrial energy applications throughtout the United States for many years. Manmade, synthetic gases are also known and used for energy and other useful purposes.
As local natural gas supplies dwindle, transported and stored liquid energy gases will become an increasingly significant source of energy throughout the world dispite the known hazards. These gases are of three basic types: natural (LNG), petroleum (LPG), and synthetic (LSG) including artificially produced domestic energy gases (i.e., methane and ethane) and industrial energy gases (i.e., acetylene and propylene). These liquified gases are expected to provide a primary source of heat producing energy for the near range future, certainly during the interim until other energy sources such as solar, geothermal and fusion are made practical and economical. Also, of the remaining, readily available energy sources (i.e., coal, oil, gasoline, uranium and gas), only the liquified energy gases burn cleanly, which renders them attractive energy source alternatives to crude oil and coal, as society becomes increasingly concerned with the prevention of air pollution.
Natural gas is a mixture of hydrocarbons, typically 65 to 99 percent methane, with smaller amounts of ethane, propane and butane. When natural gas is chilled to below minus 263 degrees Fahrenheit, it becomes an odorless, colorless liquid having a volume which is less than one six hundredth (1/600) of its volume at ambient atmospheric surface temperature and pressure. When LNG is warmed above its -263 F. boiling point, it boils (i.e., regassifies) and expands to its over six hundred times greater original volume. Thus, it will be appreciated that a 150,000 cubic meter LNG tanker ship is capable of carrying the equivalent of 3.2 billion cubic feet of natural gas.
Of the known liquid energy gases, liquid natural gas is the most difficult to handle because it is so intensely cold. Comlex handling, shipping and storage apparatus and procedures are required to prevent unwanted thermal rise in the LNG with resultant regassification. Storage vessels, whether part of LNG tanker ships or land-based, are closely analogous to giant thermos bottles with outer walls, inner walls and effective types and amounts of insulation in between.
LNG storage tanks in the United States have heretofor been built mostly above the ground with some frozen pit facilities properly characterized as mostly above the ground. Most such tanks have been enclosed by surrounding earthen dikes. Such dikes were sized and emplaced to enclose an area and volume at least as great as the storage capacity of the largest tank within the diked area. Besides the known potential hazards of explosion and inferno created by massive rupture of such tanks, a small rupture, as by a saboteur's bullet or projectile in the upper part of the sidewall could result in a stream of LNG shooting beyond the dike, thereby rendering it useless to contain the hazard of a spill and creating the consequent likelihood of explosion and fiery inferno.
Surprisingly, until recently little attention has been focused upon the ocean and its vastness as a potentially safer environment for storage facilities for liquified energy gases, including LNG. A partially submerged offshore storage tank for liquified energy gases was disclosed in the Jackson U.S. Pat. No. 3,675,431 issued July 11, 1972. That patent described an insulated tank which was prefabricated, floated to a suitable offshore site and then sunk until its submerged base rested on the floor of the sea. An upper above-the-water domed metal cylinder extended from a concrete base. Insulation lined the interior of the tank. A thin and flexible membrane inside the insulation provided the required liquid tight interior lining of the tank. The insulation lining the submerged portion of the tank was said to be thinned, so that a layer of ice formed around the outside of the concrete base when the tank was filled with liquified gas. In accordance with the invention claimed in the patent, the ice layer supposedly acted as an outer seal for the submerged concrete.
Another prior art LNG storage facility concept was disclosed in the Glazier U.S. Pat. No. 3,727,418, issued Apr. 17, 1973. The Glazier patent described having an insulated interior membrane. A balancing fluid, said to be isopentane (2-methyl butane) transferred hydrostatic pressure from surrounding ambient water to the LNG contents.
A still different approach was described in the offshore LEG storae facility disclosed in the McCabe U.S. Pat. No. 3,828,565, issued Aug. 13, 1974. Therein, an insulated buoyant tank moved telescopically up and down in a larger receiver tank containing seawater, oil or other liquid in accordance with the quantity of LEG at atmospheric pressure stored therein from time to time.
Yet another underwater storage apparatus was disclosed in the Toyama U.S. Pat. No. 3,837,310 issued Sept. 24, 1974. Therein, a torus shaped buoyancy control tank surrounded a larger spherical submerged offshore oil tank. The slight positive buoyancy was equalized over the range of oil storage capacity by the introduction or removal of water ballast from the buoyancy control tank. The Toyama facility was tethered to a base at the seabed by a plurality of cables.
Subterranean storage vessels for LNG have been used in Japan with some claimed advantages over surface, landbased storage facilities. Nevertheless, the hazards presented by such facilities, particularly from earthquake damage, remain unabated. Also inspecting and maintaining such facilities was extremely difficult and hazardous.
Another prior proposal for offshore underwater storage of crude petroleum product was described in the Pogonowski U.S. Pat. No. 3,643,447, issued Feb. 22, 1972. Therein, a frame anchored to the sea floor supported an expansible, bladder-like tank held to the frame at a predetermined depth below the surface. Crude petroleum from an undersea well was piped into the tank continuously and caused it to expand. A delivery conduit from the tank extended to the surface and delivered the crude into tanks of an awaiting barge or ship. Latent hydrostatic lifting pressure developed by sea pressure against the flexible tank was used to force the crude out of the bladder-like tank, through the conduit, and into the awaiting tanker without pumping being required. While the Pogonowski contrivance might have been feasible for storage of liquid crude at ambient sea temperatures, the use of ambient water pressure to maintain the liquid state of the liquified gas, or the use of depth in the water to dissipate small leaks from the facility without the danger of fire or explosion.
The primary direction in which offshore LNG storage has moved in the past two years has been embodied by floating moored terminals. These floating terminals offer some advantages over land based storage, i.e., isolation from population centers and minimization of contact with real property and fixed structures, minimization of effect from earthquake hazards, removal of the need to dredge in order for tankers to approach the facilities, and need for large tracts of land to be used as buffer zones. Two primary designs exist: A semisubmersible floating tank structure moored near or under a floating liquefaction plant embodied by the design set forth by a European consortium led by Linde AG of West Germany. The second design, proposed by Imodco-General Dynamics, embodies a flat bottomed, square-ended barge moored by a single point mooring system (SPM). While both of these designs overcome many of the problems inherent to land based storage as mentioned above, they still have many disadvantages. They are subject to buffeting by severe sea conditions, which results in slosh and, subsequently, in a tendency for a portion of the liquified gas to flash to a gaseons phase from internal (intermolecular) friction. They are susceptible to collision and damage from surface vessels. They are susceptible to aerial and surface assult. They are susceptible to fire and explosion due to storage in or near an atmospheric storage medium which supports combustion, and, while the facility is removed from population centers and large tracts of real property and fixed structures, the facility itself would be subject to catastrophic loss. This would surely result in the loss of the facility operations crew, the loss of a costly facility, interruption in the storage/liquefaction network, and possibly the loss of a very costly LNG transport vessel.
A radically different and vastly improved offshore storage system is disclosed in my co-pending patent application with Mark Stolowitz, co-inventor, Ser. No. 967,472, filed Dec. 7, 1978, entitled Offshore Submarine Storage Facility for Highly Chilled Liquified Gases, now U.S. Pat. No. 4,232,983. Therein, an offshore tanker terminal and submarine storage facility for chilled liquified gases included an elongated vertical frame work anchored to the sea floor. A variably ballasted insulated storage vessel formed of two slidably meshing pistons chambers with herispherical ends, moved up and down in the framework, like an elevator, so that external ambient seawater pressure available at a depth selected for the desired pressure was applied to the liquified contents of the vessel to inhibit regassification.
Since making this original invention, I have invented certain improvements which are disclosed hereinafter which render the original invention even more useful, practical and feasible for widespread adoption and use for offshore storage and handling of liquified energy gases.